Posted on Thursday, 21st June 2007 by sean
Earlier I looked at stub areas. One problem we found was that you can’t have an ASBR in a stub area — no “redistribute static” on any external links. What a pity!
Not so stubby areas get around this by allowing the ASBR to exist and propagate LSAs. The problem is that stub areas can’t have type 5 LSAs (external), so NSSAs use a type 7 LSA which is converted back to a type 5 on the ABR as the LSA is flooded to the backbone. Because of this functionality, all routers need to be configured as a NSSA.
R4#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R4(config)#router ospf 1
R4(config-router)#area 2 nssa
OSPF: Area is configured as stub area already
R4(config-router)#no area 2 stub
R4(config-router)#area 2 nssa
And on the ABR:
r3#conf t
Enter configuration commands, one per line. End with CNTL/Z.
r3(config)#router ospf 1
r3(config-router)#no area 2 stub
r3(config-router)#area 2 nssa
Like the stub area, we have lost our external route to 1.1.1.1:
R4#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
4.0.0.0/24 is subnetted, 1 subnets
S 4.4.4.0 is directly connected, Null0
10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C 10.0.2.0/24 is directly connected, FastEthernet0/0
O IA 10.0.0.0/24 [110/3] via 10.0.2.3, 00:00:27, FastEthernet0/0
O IA 10.0.1.0/24 [110/2] via 10.0.2.3, 00:00:27, FastEthernet0/0
O IA 10.0.33.1/32 [110/4] via 10.0.2.3, 00:00:27, FastEthernet0/0
Note there is no static route injected by the ABR! Another option is needed:
Now, R4 has a default route: Note it is N2, meaning it is a type 7 (pseudo type 5) LSA. Remarkably similar to the default route we saw in the stub area! Now, to make use of the nssa features. With redistribute static subnets on, R4 generates type 7 LSAs for the 4.4.4.0 prefix: And R3 sees it as a N2 route: And way over on R1, it looks like a regular old external route: There is also a "no-summary" option to creating an NSSA, which does exactly the same thing as in stub areas. How Does OSPF Generate Default Routes? No related posts. Related posts brought to you by Yet Another Related Posts Plugin. Posted in Routing | Comments (0)
r3(config-router)#area 2 nssa ?
default-information-originate Originate Type 7 default into NSSA area
no-redistribution No redistribution into this NSSA area
no-summary Do not send summary LSA into NSSA
r3(config-router)#area 2 nssa default-information-originate
O*N2 0.0.0.0/0 [110/1] via 10.0.2.3, 00:00:49, FastEthernet0/0
R4#show ip ospf database nssa-external
OSPF Router with ID (10.0.2.4) (Process ID 1)
Type-7 AS External Link States (Area 2)
Routing Bit Set on this LSA
LS age: 102
Options: (No TOS-capability, No Type 7/5 translation, DC)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 10.0.2.3
LS Seq Number: 80000001
Checksum: 0x454E
Length: 36
Network Mask: /0
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 1
Forward Address: 10.0.2.3
External Route Tag: 0
R4(config)#router ospf 1
R4(config-router)#redistribute static subnets
R4#show ip ospf database nssa-external 4.4.4.0
OSPF Router with ID (10.0.2.4) (Process ID 1)
Type-7 AS External Link States (Area 2)
LS age: 79
Options: (No TOS-capability, Type 7/5 translation, DC)
LS Type: AS External Link
Link State ID: 4.4.4.0 (External Network Number )
Advertising Router: 10.0.2.4
LS Seq Number: 80000001
Checksum: 0x367
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 10.0.2.4
External Route Tag: 0
r3#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
1.0.0.0/24 is subnetted, 1 subnets
O E2 1.1.1.0 [110/10] via 10.0.1.2, 00:02:13, FastEthernet0/0
4.0.0.0/24 is subnetted, 1 subnets
O N2 4.4.4.0 [110/20] via 10.0.2.4, 00:00:08, FastEthernet1/0
9.0.0.0/24 is subnetted, 1 subnets
S 9.9.9.0 is directly connected, Null0
10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C 10.0.2.0/24 is directly connected, FastEthernet1/0
O IA 10.0.0.0/24 [110/2] via 10.0.1.2, 00:02:13, FastEthernet0/0
C 10.0.1.0/24 is directly connected, FastEthernet0/0
O IA 10.0.33.1/32 [110/3] via 10.0.1.2, 00:02:13, FastEthernet0/0
r3#
r1>show ip route 4.4.4.0
Routing entry for 4.4.4.0/24
Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 3
Last update from 10.0.0.2 on FastEthernet0/0, 00:01:11 ago
Routing Descriptor Blocks:
* 10.0.0.2, from 10.0.2.3, 00:01:11 ago, via FastEthernet0/0
Route metric is 20, traffic share count is 1
r1>show ip ospf data
r1>show ip ospf database e
r1>show ip ospf database external 4.4.4.0
OSPF Router with ID (10.0.33.1) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSA
LS age: 87
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 4.4.4.0 (External Network Number )
Advertising Router: 10.0.2.3
LS Seq Number: 80000001
Checksum: 0x9DD7
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 10.0.2.4
External Route Tag: 0
OSPF Not-So-Stubby Area (NSSA)
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